Your thoughts of and experience with drop tubes

[MCMXI]

I'm in the process of working up a load for the rifle shown in the thread here and was discussing drop tubes with a coworker because I'm working with compressed loads. The general thought is that a long drop tube allows a few more grains of powder in the case before the load is compressed. In addition, the more ordered packing of kernels can lower ES and SD numbers which can often be correlated to smaller groups at long range. I ordered a couple of drop tubes from Midway and borrowed my coworker's drop tube last night to run a very small test. Both cases shown in the photo below are new .308 Win Lapua and both are shown with 50.8gr of Varget. The case on the left was filled using a .30 cal Satern funnel whereas the case on the right was filled using an MTM funnel with a 4" drop tube. Clearly the case on the right has kernels more densely packed so what's the physics here? Any thoughts?

http://www.midwayusa.com/product/613906/mtm-universal-powder-funnel-set-17-to-45-caliber

http://www.midwayusa.com/product/293879/forster-blue-ribbon-powder-funnel-with-long-drop-tube

 

[j1]

I used old aluminum arrows as drop tubes. They cost nothing and can be made in varying lengths to suit your needs. Worked very well with Hornet cases.

 

[Reloadron]

Haven't used them for some time but used the 4" and 6" powder drop tubes extensively for compressed loads when I was shooting 6mm PPC.



There is no magig as the powder has a longer drop allowing gravity to do its thing. The powder accelerates as it drops down the tube. These tubes are great for doing compressed or near compressed loads and they beat tapping a filled case on the bench top to get the powder to settle. Also, as J1 points out they can be store bought or home brew.

Ron

 

[Walkalong]

I have used two types, and the drop tubes that are a small diameter all the way down compact powder much better than the ones that have the large diameter tube and then a small opening at the bottom like those Ron pictured. Those work OK, but the ones with a small diameter for the whole length are significantly better. They are hard to find though.

What is the inner diameter of that Forster tube MCMXI?

what's the physics here?

Other than the fact the case is filled much slower, I have no idea.

 

[cfullgraf]

I have a set of drop tubes similar, or the same, as the ones shown by Reloadron.

Even with the larger internal diameter, I find they help when charging 17 Remington cases with stick powders. The charge fits in the case better when I use the drop tube.

I do have to operate the powder measure more slowly, essentially pouring, not dumping, the powder into the case. Otherwise, the powder bridges in the drop tube.

Bridging seems to be less of a problem with the larger diameter of the opening in the case mouth adapters that can be used with cases for larger caliber cartridges.

It would be interesting to try a drop tube with a smaller internal diameter.

I suspect the physics behind it is the sticks of powders get oriented in the same direction so that they pack with a higher density.

 

[flashhole]

I've used drop tubes in the past and a few of my loads are compressed. When I really want to settle powder I use the electric toothbrush body shown below. Running the flat on the shaft up and down the case really packs the powder in.

 

[Walkalong]

http://www.sinclairintl.com/reloadi...bes/sinclair-custom-drop-tubes-prod33706.aspx

I haven't used the Sinclair, but they should work well. The more compacted you want the powder, the smaller the ID and the longer the tube. (Some powders won't work in the thin tubes.)

 

[Laphroaig]

what's the physics here?

Hmm, that's a good question. My guess that the powder grains have more kinetic energy when they fall into the case thereby allowing them to get a denser packing arrangement. Similar to flashholes toothbrush but from the get-go.

Laphroaig

 

[flashhole]

This is my rationale. Force = mass * acceleration.

Longer drop tube makes for greater acceleration thus greater force and the powder packs more densely.

 

[MCMXI]

What is the inner diameter of that Forster tube MCMXI?

Walkalong, I don't know but will certainly measure it when it arrives on Monday.

There is no magig as the powder has a longer drop allowing gravity to do its thing. The powder accelerates as it drops down the tube.

I suspect the physics behind it is the sticks of powders get oriented in the same direction so that they pack with a higher density.

Yes, these are the two prevailing theories that I've heard. It shouldn't be too hard to figure out the difference in velocity of a kernel falling into a case through a 4" drop tube vs. a simple funnel. The alignment theory would require a high speed camera to see if the kernals align vertically as they fall.

I do have to operate the powder measure more slowly, essentially pouring, not dumping, the powder into the case. Otherwise, the powder bridges in the drop tube.

The other thought I had somewhat along the lines of the comment above is the difference in the rate at which the powder falls into the case. This would be easy to test by simply pouring powder quickly and slowly using the same funnel or drop tube.

 

[Reloadron]

I can share this much. The tubes I posted have an ID of .386" and were bought from Sinclair about 25 years ago. The actual ID at the case mouth will depend on the case mouth adapter used. The tubes mount directly to my old RCBS Uniflow powder dispenser.

When I used them I just developed a sort of rhythm with the Uniflow handle. Seldom if ever did I have an issue with the powder bridging and I threw quite a bit of stick powder.

Ron

 

[spitballer]

I've never used a drop tube but FWIW I've noticed that trickling powder through the funnel results in lower volume, as opposed to dumping it in quickly. Not sure why this would be...

 

[Stony]

I have played with drop tubes over the years, but got away from them. I sort of follow the same theory as Flashtube above, and vibrate the powder in the case. I have a small vibrating table and holding a case on it for a few seconds, you can see the powder settle down into the case. I normally just use this method for black powder, but I think it's more effective at settling the powder into the case than a drop tube.

 

[MCMXI]

What is the inner diameter of that Forster tube MCMXI?

Walkalong, the funnel arrive today. The I.D. of the tube is .243", the flare at the end has an O.D. of .435" and the tube is about 5-3/4" long measured from the bottom of the funnel where it transitions into the neck.

 

[flashhole]

Nice piece of gear, thanks for sharing.

 

[Walkalong]

It will be interesting to see how it does.

 

[MCMXI]

Another update. I filled up a new .308 Win Lapua case use a Satern .308 funnel and got as much powder in the case as I could (about 58gr). I then dumped the powder in a pan and refilled the same case using the drop tube. My method wasn't exactly rigorous but I'm estimating that I'll be able to get an additional 1.5gr to 2.0gr of powder in the case at the point where the load is just starting to be compressed. This could be significant ... or not.



I didn't care for all the static in the plastic funnel so I cut some threads into a 6.5mm Satern funnel and moved the tube over.